Ph.D. thesis (pdf) - dirac

168 Boson Peak
perature effect on the alpha relaxation. We stress that the interpretation above is
a suggestion based on the very limited knowledge of the pressure dependence of the
boson peak intensity. More studies would be needed to verify this interpretation.
It is clear that the correlation, if present at all, is a trend rather than a one to one
correlation between fragility and R. The most prominent outlier in figure 8.11 and
particularly in figure 8.12 is sorbitol. In figure 8.20 we show the S(ω) of sorbitol
at T g . The peak is remarkably different from the boson peak in the other systems
we have studied. The boson peak energy is a factor 2 larger, and the shape is also
distinctively different. Sorbitol is a hydrogen bonded system. This suggests that the
boson peak is different in a hydrogen bonded system as compared to other molecular
liquids.
8.4 Summary
This chapter has two main sections. The first section contains a study of the boson
peak under pressure in glassy PIB (M w =3580g/mol). We find i) that the boson peak
energy increases faster with pressure than any of the other modes. ii) that intensity
of g(ω)/g D (ω) increases with pressure. iii) the shape of the boson peak seen in the
rDOS g(ω)/ω 2 is unaffected by pressure.
The decrease in the low frequency intensity seen directly in the rDOS (figure 8.5) has
earlier been used to favor the interpretation that the shift in the boson peak is due
to a suppression of specific modes rather than a shift of modes to higher frequencies
[Gurevich et al., 2005; Hizhnyakov et al., 2000; Klinger, 1999, 2001; Inamura et al.,
2000, 2001]. Our results are in clear contradiction with this view, as we do not see
a suppression of modes.
It has recently been suggested by Monaco et al. [2006 b] that the changes in boson
peak due to pressure could be explained entirely by the pressure induced increase in
the macroscopic force constants, and that they would disappear when normalizing
to the Debye frequency and the amplitude of the Debye density of states. This is
clearly not the case for the pressure dependence of the boson peak in PIB.
The second part of this chapter deals with proposed correlation between the fragility
and the parameter R. This parameter is given by the intensity of the quasi-elastic
scattering (plus the Debye level) relative to that of the boson peak evaluated at T g .
We test this correlation on a set of samples which span a large range in isobaric as
well as isochoric fragility. In both cases we find a somewhat scattered correlation. We
moreover find that the parameter R is constant along the glass transition isochrone